1. Field of the Invention
The invention relates to circuits using magnetoresistive elements as magnetic transducers.
2. Description of the Prior Art
Magnetoresistive elements exhibiting a resistance dependent upon the intercepted magnetic flux have been biased by alternating current for a variety of purposes. In McCoubrey U.S. Pat. No. 2,571,915, an alternating magnetic field is applied to a magnetoresistive element to form a DC amplifier. A bridge arrangement of magnetoresistive elements uses an alternating current to achieve magnetic readback in Wiegand U.S. Pat. No. 2,918,534 and in de Koster U.S. Pat. No. RE26,610. The principles involved in biasing a magnetoresistive readback head with alternating current are explained by Sua-For Sun in the IEEE TRANSACTIONS ON AUDIO, Vol. AU-13 No. 2, pages 41-43. All of these devices exhibit deleterious signal drift, noise and instability due to the sensitivity of the magnetoresistive elements to temperature and current changes, power supply variations, media-head relationships, etc. While inductive heads are subject to a variety of different problems which subtract from the scope of their use, the inductive element is, nevertheless, functional under a wide variety of conditions than is the magnetoresistive element. A significant characteristic of a magnetoresistive element which is adversely affected by unpredictable variations is the element's signal amplitude. Control of an output amplitude as a function of the difference between the actual amplitude and a desired amplitude value is generally well known; for example, in the automatic control of the audio output volume of a broadcast receiver. Copending application Ser. No. 684,053, "Dynamic Transducer Biasing Signal Amplifying Circuitry", by J. D. Harr, filed May 6, 1976, assigned to International Business Machines Corporation, discloses a circuit for adjusting the DC bias of a magnetoresistive element to maintain linear operation as a function of an output.
The prior art discussed does not address the peculiar problems of maintaining linear operation of a readback transducer comprising magnetoresistive elements biased by alternating current. For example, the bridge-arranged magnetoresistive elements shown in the IBM TECHNICAL DISCLOSURE BULLETIN, Vol. 19, No. 3, August, 1976, page 789, exhibits an output signal envelope amplitude which significantly and unpredictably varies undesirably as a function of the previously listed variables. A signal, for controlling the amplitude as described in the referenced application Ser. No. 684,053, is not available because there is no DC output and there is no bias amplitude control.